3rd generation (3G) systems, such as the Universal Mobile Telecommunication System (UMTS) have been developed and deployed to further enhance the communication services provided to mobile users compared to those communication services provided by the 2nd generation (2G) communication system known as the Global System for Mobile communication (GSM).
High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) have been developed to optimise UMTS with increased data rate and capacity for packet data services in downlink and uplink, respectively. HSDPA and HSUPA are referred together as High Speed Packet Access (HSPA). Standards for HSPA have been established within the Third Generation Partnership Project (3GPP): HSDPA was introduced as a release 5 feature in 3GPP and HSUPA was introduced in release 6. Within 3GPP release 7, further improvements to HSPA have been specified in the context of HSPA+ or HSPA evolution.
As is well known, cellular communication systems, such as UMTS, provide communication to mobile devices via a plurality of cells, with each cell served by one or more base stations. The base stations are interconnected by a radio network controller which can communicate data between the base stations. A mobile device communicates via a radio communication link with a base station of the cell within which the mobile station is situated. In UMTS, the base stations which are part of the UTRAN are known as Node Bs and a mobile device is known as User Equipment (UE).
3GPP release 7 introduced Continuous Packet Connectivity (CPC) which is aimed at providing improved user experience. For example, CPC allows a UE to stay connected over longer periods and so avoids frequent connection termination and re-establishment, even though the UE may only occasionally have active periods of data transmission. CPC includes features to reduce the uplink and downlink control channel overhead so as to reduce uplink noise, reduce power consumption in the UE (and so preserve UE battery) and increase downlink capacity. These features include an Uplink Discontinuous Transmission (DTX) mode for reducing uplink control channel overhead and a Downlink Discontinuous Reception (DRX) mode for reducing downlink control channel overhead. During the DTX and DRX modes (i.e. during temporary HSPA data transfer inactivity periods), the UE is in an active state known as a DCH state. The 3GPP Technical Specification TS 25.308 V7.12.0 gives an overview of CPC (see section 11) and 3GPP Technical Specification TS 25.214 V7.17.0 provides details of the physical layer impacts of CPC (section 6C).
The communications network configures the UE with DTX/DRX parameters which allow the UE to enter the DTX and DRX modes and which define the patterns or cycles of the DTX and DRX modes.
If the UE has no data to transmit, the UE enters a DTX mode during which the UE automatically stops the continuous transmission of control information on the Uplink Dedicated Physical Control Channel (UL-DPCCH) and applies a known DTX pattern for transmission of control information on UL-DPCCH. Typically, the DTX mode includes two DTX patterns or cycles configured by the communications network: one short pattern or cycle (DTX1) and one long pattern or cycle (DTX2). The pattern defines the length of the transmission cycle, the number of slot and slot position in the transmission cycle that the UE shall transmit uplink DPCCH (UL-DPCCH). The choice of the pattern depends on data activity and is determined by the UE. The UE will initially switch to the short pattern when there is no data to be transmitted and then after a certain period of time and when there is still no data to be sent, the UE will switch to the long pattern. In one example, for the long pattern DTX2, the UE transmits control messages on UL-DPCCH for 2 ms every 320 ms. The UL-DPCCH when received is used by the network to evaluate UE transmit power and generate downlink Transmit Power Control commands (DL-TPC) in order to control the transmission power of the UE. The control information on UL-DPCCH also include uplink Transmit Power Control (UL-TPC) commands or bits which are used by the communications network to control its downlink transmission power towards the UE.
When in the DTX mode, the UE does not send uplink Transmit Power Control (UL-TPC) commands in all uplink slots. This means that it takes longer for the communications network to react which results in the downlink power control being slower. Thus, when a UE operates in a CPC DTX mode, there is a higher risk of de-synchronisation in the downlink.
The downlink TPC (DL-TPC) commands are only transmitted by the communications network when it has received an UL-DPCCH from the UE. This means that it takes longer for the communications network to react which results in the uplink power control also being slower. Thus, when a UE operates in a CPC DTX mode, there is a higher risk of de-synchronisation in the uplink and added unwanted uplink interference (as the power is not correctly adjusted).
With the higher risk of de-synchronisation in the downlink and uplink, there is a higher risk that the connection will be lost. Indeed, the UE is monitoring the DPCH quality in order to detect a loss of the signal at a physical level (as specified in TS 25.224). Thresholds Qout and Qin specify respectively at what DPCH quality levels the UE shall shut its power off and when it shall turn its power on. In CPC mode, with the higher risk of de-synchronisation, the UE can more easily enter a state where it has shut down its transmitter because the threshold Qout has been reached.
US patent application no. 2009/0086682 describes a mechanism for downlink out-of-sync detection in CPC which requires the serving Node B to send valid TPC bits even while the UE is in DTX mode. By continuously transmitting TPC bits to all UEs in a cell, such a mechanism increases the amount of interference.